STTMRAM element having multiple perpendicular MTJs coupled in series

Abstract

The present invention is directed to a multi-state current-switching magnetic memory element configured to store a state by current flowing therethrough to switch the state. The magnetic memory element includes a stack of two or more magnetic tunneling junctions (MTJs) with each MTJ including a free layer with a switchable magnetic orientation perpendicular to a layer plane thereof, a barrier layer, and a fixed layer with a fixed magnetic orientation perpendicular to a layer plane thereof. Each MTJ is separated from other MTJs in the stack by at least an isolation layer. The stack of MTJs may store more than one bit of information. The free layer of each MTJ has a switching current threshold different from free layers of other MTJs in the stack.

What is claimed is: 
     
       1. A multi-state current-switching magnetic memory element configured to store a state by current flowing therethrough to switch the state comprising:
 a stack of two or more magnetic tunnel junctions (MTJs), each MTJ including a free layer with a switchable magnetic orientation perpendicular to a layer plane thereof, a barrier layer, and a fixed layer with a fixed magnetic orientation perpendicular to a layer plane thereof, each MTJ being separated from other MTJs in the stack by at least an isolation layer, the stack of MTJs operable to store more than one bit of information, 
 wherein the free layer of each MTJ has a switching current threshold different from free layers of other MTJs in the stack. 
 
     
     
       2. The magnetic memory element of  claim 1 , wherein the free layer of each MTJ has a different composition from free layers of other MTJs in the stack. 
     
     
       3. The magnetic memory element of  claim 1 , wherein the free layer of each MTJ has a different oxide content from free layers of other MTJs in the stack. 
     
     
       4. The magnetic memory element of  claim 1 , wherein the isolation layer has an amorphous structure. 
     
     
       5. The magnetic memory element of  claim 1 , wherein the isolation layer is non-magnetic. 
     
     
       6. The magnetic memory element of  claim 1 , wherein each MTJ in the stack stores one bit of information. 
     
     
       7. The magnetic memory element of  claim 1 , wherein each MTJ has a different thickness from other MTJs in the stack. 
     
     
       8. The magnetic memory element of  claim 1 , wherein each MTJ has a different in-plane dimension compared with other MTJs in the stack. 
     
     
       9. The magnetic memory element of  claim 1 , wherein the barrier layer of each MTJ has a different thickness from barrier layers of other MTJs in the stack, thereby providing each MTJ a unique electrical resistance.